I have a dream that one day every valley shall be exalted, every hill and mountain shall be made low, the rough places will be made plain, and the crooked places will be made straight, and the glory of the Lord shall be revealed, and all flesh shall see it together.
This quote references Isaiah 40 (which is also quoted in Luke 3 in reference to John the Baptist)
Comfort, comfort my people,
says your God.
Speak tenderly to Jerusalem,
and proclaim to her
that her hard service has been completed,
that her sin has been paid for,
that she has received from the LORD’s hand
double for all her sins.
A voice of one calling:
“In the desert prepare
the way for the LORD;
make straight in the wilderness
a highway for our God.
Every valley shall be raised up,
every mountain and hill made low;
the rough ground shall become level,
the rugged places a plain.
And the glory of the LORD will be revealed,
and all mankind together will see it.
For the mouth of the LORD has spoken.”